After years in which it insisted that it had no intention of competing directly with Microsoft, Google is parking its tanks front and centre on Microsoft's lawns. The Chromebook - a "thin client" laptop that will have virtually no local storage - launches today, Wednesday, and is the first serious challenge to the Windows/Office hegemony in two decades.

The Chromebook doesn't run Windows; it runs Google's second operating system (after Android), ChromeOS, which is stripped down so far that it just runs a browser and associated programs. No Windows. No Microsoft Office. No Microsoft anything, in fact. ChromeOS is a Linux-based system that Google has been working on since 2009. And now it's becoming real, through "Chromebooks" from Samsung and Acer (and also, unofficially, from the Australian company Kogan, which is selling them in Australia and the UK).

Yet to begin with, the threat to Microsoft, and the promise to Google, from Chrome is minimal. First, the Chromebook is not aimed at the average consumer. It's aimed at the enterprise customers who are Microsoft's bread and butter. Google, with the Chromebook, is aiming to eat Microsoft's lunch - and its dinner and breakfast too. How? By nibbling away at Windows and Office revenues by finding the companies that have finally had enough of Office upgrades, and aren't that keen on the Windows upgrade cycle - and associated costs - either. The other target: schools - where Microsoft first gets its customers.

Chrome is, make no mistake, a dagger poised over Microsoft's heart. The only question is whether it's made of steel or rubber.

Google claims that the total cost of ownership (TCO) for ChromeBooks is 50% less than for an equivalent Windows PC - about $1,500 per Chromebook compared to the estimated $3,000 per PC figure that Gartner puts on it (which includes the cost of the machine, software, and especially of support). "We've corroborated that $3,000 figure with enterprise customers," said Sundar Pichal, the senior vice-president at Google of the Chrome project.

I asked the team what their targets are for sales; they replied that they don't have any (which I don't believe
that
; you've got to have targets because otherwise the manufacturers won't take the contract, and Google works to targets all the time internally). They said they're more interested in "how much users are using it, and how much they like it. If we focus on that, then more and more units will ship."

In case you haven't had the memo, here's the detail about the Chromebooks:• 12.1 in laptops• 16GB SSD storage (the Kogan models offer 30GB)• 2GB RAM (1GB on the Kogan model)• Intel Atom 1.66GHz • made by Samsung or Acer (or, unofficially, Kogan)

OK - so far, so like a big-screened netbook. And now some more detail: • 8-second boot time• 6-week update cycle for the OS and the browser• will be available in the US, UK, France, Germany, Netherlands, Spain and Italy from 15 June• price in the UK: £349 for Wi-Fi model, £399 for 3G-enabled version. (Kogan: around £225).

Essentially you can have some local apps that sort of run through the browser, but Google's idea is that you'll do everything through the browser. It says that it already has companies interested in going with it.

"Obviously", says the web page, "you're going to need a wireless network, be willing to use it subject to the provider's terms and conditions, and be ready to put up with its real life limitations including, for example, [the network's] speed and availability. When you do not have network access, functionality that depends on it will not be available."

Google doesn't really have its eyes on consumers - though if they happen along then it will happily lap them up. The real people it wants to take these laptops up are the companies that are already using Google Apps for Your Domain (GAYD) to run things such as Google Docs and Spreadsheets. Yes, it's absolutely true that Google Docs isn't a patch on Microsoft Office if you need to do something complicated. But lots of companies and lots of staff don't need to do anything complicated. And they are the low-hanging fruit that Google is after.

To those enterprises spending $3,000 per year on a PC, once you factor in the cost of helpdesks, antivirus, firewalls, backups and so on, Google says: we'll rent you the machines, including support, for $28 per month per unit (in the US; UK prices not yet announced), or $20 per unit for students. That works out to $336 per unit per year for business or $240 for education.

For that, Google's argument is that you don't need to bother about antivirus (argue about that later) or firewalls, or backups. The thing about a Chromebook, Google says? Destroy one, and all you need to do is log in to a new one. All your data is still there. If a corporate one gets lost or stolen, there's no data on the hard drive to get stolen, and no way to break into the cloud-stored data. All that's lost is a bit of easily replaced hardware.

For enterprises, this is a stunning pitch. Yes, many people find the idea of cloud storage hard to believe. But it's entirely possible to shift businesses - and schools - over to it in many cases. (Disclosure: the Guardian uses Google Apps. Other UK national newspapers, including the Telegraph Media Group, do too.)

Where's the profit for Google? "Users who use them use the web more," says Pichal. "And where people use the web more, it's great for Google. That's on the consumer side; then our profits improve. On the business and school side, it's a clear revenue business. So we hope to have a successful business."

Companies don't even have to use GAYD: "we debated this quite a bit," says Pichal. "But we found that in many cases customers don't want Google Apps but do want this, say if they're in retail or call centres, where they're using a helpdesk app." (Imagine a giant call centre switching from Windows to ChromeOS. That's a vision to chill a Microsoftie's blood.)

Now let's get down to the numbers. Google says it has 3m businesses using GAYD, and more than 30m business users actively using Google Apps every day. Businesses replace their desktop machines roughly every three years, laptops every two years. Business sales make up around half of the 350m PCs sold every year. The key question then is how many of those are in the "mature markets" - North America and Europe - and so might be targeted in the longer term by ChromeOS.

Fortunately, we have those numbers, provided by IDC: 54.9m desktops this year, 105.4m laptops, of which half of each are businesses: so 27m desktops, 52m laptops being bought this year will be replacements in the business market. (This is in an installed base in the mature markets of 740m PCs.)

Initially, you wouldn't expect many businesses to put Chrome systems in; it'll be experimental. But once they're in, that's revenue that's lost to Microsoft both from Windows and from Office, and the latter could be the more painful loss over the longer term.

But even if all 30m GAYD-using users were to switch over to ChromeOS (which isn't going to happen, because lots of those businesses use their PCs for functions that can't be done through the browser; they need native Windows apps), it wouldn't have a big impact at first on Microsoft. 30m PCs represents about $1.69bn of revenue and $1.2bn of profit for the Windows division, or about one-third of one quarter's revenue and profits. And that's if every single GAYD customer shifts over.

So ChromeOS isn't an immediate, or perhaps even medium-term threat to Windows. A bigger threat, in fact, is the fact that it is now the emerging markets such as China which are driving PC sales growth, and Microsoft has found that piracy there reduces its revenues to 5% of what it gets in the West. That bothers Steve Ballmer far more.

The danger though will be in the longer term if consumers start to pick up the idea that Chromebooks are nifty, and if big PC sites such as call centres do start using them because of their low price and replaceability.. Here's where the simplicity/antivirus arguments might kick in. ChromeOS is allegedly virus-free because on boot up if runs a self-check called Verified Boot, which can detect corruption in the software, and repair it as necessary. The browser tabs all run in their own sandbox, so nothing should escape from there anyway.

This is of course enormously disruptive to the security companies that have turned antivirus protection on Windows PCs into an industry worth tens of billions of dollars. The effect on them will arguably be much greater if ChromeOS gets any traction inside businesses. Of course. ChromeOS won't stop people from clicking on phishing sites, but it should shortcut many of the simplest scams out there. And the fact that everything gets funnelled through Google's servers, which will scan for malware of all sorts, suggests that the real benefits may come from the level of protection far more than anything else.